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Bacterial Staining Techniques, Lecture notes of Microbiology

the different method for bacterial staining

Typology: Lecture notes

2012/2013

Uploaded on 10/03/2013

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LAB 2
Bacterial Staining Techniques I
I. Complete Lab 1
II. Staining Microorganisms: Simple Stains (Direct and Negative)
III. Morphological Unknown
I. Complete Lab 1:
Collect your plates from the trays on the side bench. Observe the TSA plates for colonies of various
sizes, shapes and colors. Each bacterial or fungal species gives a characteristic colony color and
morphology. Draw the colonies observed on both TSA plates in the spaces provided in the Results
section of Lab #1. Pick three colonies from either of the TSA plates and describe the colony color and
morphology. Also observe the cloudiness (turbidity) of your nutrient broth tube and estimate the number
of bacteria per mL (see turbidity table below).
TERMS AND DEFINITIONS
Colony: a single cell divides exponentially forming a small, visible collection of cells. Colonies are
observed when bacteria are grown on a solid medium. Each colony usually contains 107-108 bacteria.
Colony morphology: Characteristics of a colony such as shape, edge, elevation, color and texture.
Turbidity: cloudy appearance of a liquid medium due to the presence of bacteria. You can "estimate"
the number of bacteria per mL by using the table below.
Turbidity # Bacteria per mL
none 0 - 106
light 107
moderate 108
*heavy 109
*Usually bacterial populations do not exceed 3 x 109 bacteria/mL when grown in liquid media.
II. STAINING MICROORGANISMS
A. Smear preparation
Simple Stains:
B. Direct stain
C. Negative stain
TERMS AND DEFINITIONS
Chromophores: Groups with conjugated double bonds that give the dye its color.
Direct, cationic, basic or positive dyes: contain positively charged groups. Examples include methylene
blue, basic fuchsin, and crystal violet. These dyes directly bind to and stain the negatively charged
surface of bacterial cells.
Negative, anionic, or acidic dyes: contain functional groups that have a negative charge. Examples
include eosin, nigrosin and Congo red. These dyes are repelled by the negatively charged surface of
bacterial cells. Thus, they stain the background, leaving the bacterial cells clear and bright against a dark
background.
Heat Fixation: application of heat to a bacterial smear preparation. This procedure simultaneously kills
and attaches the bacteria to the slide.
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LAB 2

Bacterial Staining Techniques I I. Complete Lab 1 II. Staining Microorganisms: Simple Stains (Direct and Negative) III. Morphological Unknown I. Complete Lab 1: Collect your plates from the trays on the side bench. Observe the TSA plates for colonies of various sizes, shapes and colors. Each bacterial or fungal species gives a characteristic colony color and morphology. Draw the colonies observed on both TSA plates in the spaces provided in the Results section of Lab #1. Pick three colonies from either of the TSA plates and describe the colony color and morphology. Also observe the cloudiness (turbidity) of your nutrient broth tube and estimate the number of bacteria per mL (see turbidity table below). TERMS AND DEFINITIONS Colony : a single cell divides exponentially forming a small, visible collection of cells. Colonies are observed when bacteria are grown on a solid medium. Each colony usually contains 10^7 - 108 bacteria. Colony morphology : Characteristics of a colony such as shape, edge, elevation, color and texture. Turbidity : cloudy appearance of a liquid medium due to the presence of bacteria. You can "estimate" the number of bacteria per mL by using the table below. Turbidity # Bacteria per mL none 0 - 106 light 107 moderate 108 *heavy 109 *Usually bacterial populations do not exceed 3 x 10^9 bacteria/mL when grown in liquid media. II. STAINING MICROORGANISMS A. Smear preparation Simple Stains: B. Direct stain C. Negative stain TERMS AND DEFINITIONS Chromophores : Groups with conjugated double bonds that give the dye its color. Direct , cationic , basic or positive dyes: contain positively charged groups. Examples include methylene blue, basic fuchsin, and crystal violet. These dyes directly bind to and stain the negatively charged surface of bacterial cells. Negative, anionic, or acidic dyes: contain functional groups that have a negative charge. Examples include eosin, nigrosin and Congo red. These dyes are repelled by the negatively charged surface of bacterial cells. Thus, they stain the background, leaving the bacterial cells clear and bright against a dark background. Heat Fixation: application of heat to a bacterial smear preparation. This procedure simultaneously kills and attaches the bacteria to the slide.

MICROBIOLOGIST'S TOOLS

A. Smear Preparation The first step in most bacterial staining procedures is the preparation of a smear. In a smear preparation, cells from a culture are spread in a thin film over a small area of a microscope slide, dried, and then fixed to the slide by heating or other chemical fixatives. A good smear preparation should be...

  1. A thin layer of cells so that individual cells can be observed.
  2. Fixed appropriately to allow repeated washings during staining. PROCEDURE: (EACH STUDENT) Note: A good smear preparation is the key to a high-quality stain. Care taken when creating a smear will allow for accurate observations.
  3. Use a slide from your slide box. If necessary, clean the slide using soap and water. Dry the slide using a KimWipe. Place the frosted side of the slide facing up and draw a circle (about the size of a nickel) on the bottom (unfrosted) side of the slide. Place 2-3 loopfuls of water on the slide. Don’t forget to draw a focus line on the top of the slide.
  4. Flame an inoculating needle and allow it to cool. Pick up a "tiny" amount of an Escherichia coli colony and mix it into the drop of water on the slide.

f. After examining the slide - move the oil immersion objective away from the slide. Clean the objective thoroughly with lens paper (NOT KimWipes!) and lens cleaning solution.

  1. Draw the organisms observed in the microscopic field - record in Results Lab 2. Note : Saccharomyces cerevisiae is a species of yeast. It is a relatively large single-celled eucaryotic organism. Escherichia coli is a "tiny" rod shaped bacteria (procaryotic). C. Negative Stain In contrast to direct stains that bind to bacteria directly, a negative stain colors the background of a smear rather than the bacteria. These stains have negatively charged functional groups so they will not bind directly to negatively charged bacteria. The advantages of negative staining are: 1) bacteria are not heat fixed so they don't shrink, and 2) some bacterial species resist basic stains ( Mycobacterium ) and one way they can be visualized is with the negative stain. However, negative staining does not differentiate bacteria; one can only determine morphology. PROCEDURE: (EACH STUDENT)
  2. Using a flamed inoculating loop, place 2-3 loopfuls of Congo Red in two separate circles on a clean slide. There is no need to add water to the Congo Red. B. subtilis tooth scraping Congo Red
  3. Using a flamed inoculating NEEDLE, pick up a small amount of Bacillus subtilis and stir it into one drop of Congo Red.
  4. Use a toothpick to scrape material from your teeth near the gumline and stir this into the second drop of Congo Red. Be sure to keep the two drops separate.
  5. Air dry - DO NOT HEAT FIX.
  6. Flood the slide with acid-alcohol (95% ethanol, 3% HCl) until it turns blue. This generally takes ~ 2 seconds. Drain the excess acid-alcohol into the appropriately labeled waste container but do not wash the slide.
  7. Allow the slide to air dry; do not blot.
  8. Examine both smears. First focus using the 10X objective. You will not be able to see individual organisms, but you should be able to focus on the stain. Then move to 40X and finally to the oil immersion lens with oil. Note : Organisms appear white (colorless) against a blue stained background. Draw a typical microscopic field for each slide in the Results section of this lab.

III. Morphological Unknown The staining procedures introduced in Labs 2-4 are commonly used by microbiologists to help characterize and identify bacteria. These stains often make it possible to determine the group of organisms to which an unknown isolate belongs. With few exceptions, staining is the first step in identifying a bacterial unknown. Although staining alone does not give sufficient information about the organism to make a definitive identification, it will give some important clues. You will be given an unknown pure culture on which you will perform the various stains as you go through labs 2-4. PROCEDURE: (EACH STUDENT)

  1. Collect an unknown from the side bench. Record the number of your unknown in the Results section. Your T.A. will also record the number of your unknown. It is important that the same unknown number is used throughout the identification process.
  2. Perform a direct stain (methylene blue) on your unknown. Determine the shape of your unknown and any distinctive arrangements of the cells. Record your observations in the results section following Lab 4.

LAB 2 RESULTS

II. SIMPLE STAINS

Direct Stain: Draw and label examples of Escherichia coli and S. cerevisiae. Be sure to illustrate the relative sizes of each microorganism. Negative Stain QUESTIONS:

  1. What is the purpose of simple staining? __________________________________________ _____________________________________________________________________________ _____________________________________________________________________________
  2. Differentiate between basic and acidic dyes._______________________________________


  3. What is the purpose of heat fixation? ___________________________________________

  1. Is heat fixation done for all stains? Explain. _______________________________________

  2. What is the major difference between Saccharomyces cerevisiae and Escherichia coli?

  3. When is negative staining used? ________________________________________________

    **HELPFUL HINT** 

You may find it useful to make index cards for each organism, staining procedure, and media encountered throughout this course. At this point, you can record the Gram reaction, morphology, and arrangement for each organism. Later in the course, it will be possible to add further descriptions. The course web site has several features that will assist you in this task. Although it is common practice to abbreviate the genus by using the first letter (e.g. E. coli for Escherichia coli ), for the purpose of this class you will need to write out the entire genus and species name on practical exams and quizzes. The first letter of the genus is always capitalized, but the species is not. Also, the genus and species name is always typed in italic or underlined. Organisms introduced in this lab: Escherichia coli : G- bacilli (rod) Saccharomyces cerevisiae : (yeast) Bacillus subtilis : G+ bacilli